Polyolefin pipe, and PEX in particular, currently in wide use, is flexible but may degrade by reaction with oxidizing agents, and diffusion of oxygen through the polyolefin, a combination, which over time, causes oxidative degradation. CPVC and/or blends of CPVC with less than 25% by weight of PVC (reference to “CPVC/PVC” specifies either CPVC or a blend of CPVC with <25% PVC, conventionally compounded; reference to “CPVC” specifies CPVC as conventionally compounded), optionally blended with another miscible polymer (other than PVC) in an amount insufficient to substantially change the flexibility of pipe made with CPVC or the CPVC/PVC blend, are highly resistant to oxidative degradation, and to attack by other aggressive chemicals (“chemical attack”), but is rigid when extruded as pipe. Rigidity is why piping made from highly degradation-resistant synthetic resinous materials or “plastics” such as CPVC/PVC is typically not used in domestic water distribution systems which require bending the pipe, because the combination of forces required to bend the pipe is likely to crack the pipe. To provide greater flexibility, plasticizer may be added to the blend, but toxicity requirements and long term performance will be affected.
Though rigid, sections of CPVC/PVC pipe and the appropriate fittings may be economically cemented together because they are solvent-cementable. A piping system so constructed performs its function at a level far in excess of the performance requirements of domestic water systems since the temperature of the water is relatively low, typically less than 100° C., and the pressure of the water is no more than about 790 kPa (100 psig). In particular, CPVC/PVC piping is negligibly susceptible to diffusion of oxygen and reaction with oxidizing agents, either from within or from outside the pipe.
From the foregoing, it is evident that the advantages of a PEX piping system are not available in a CPVC/PVC piping system; and, vice versa. Accordingly, much effort has been devoted to producing plastic pipe which has the advantages of both systems and the drawbacks of neither. However, any attempt to bond either CPVC/PVC to a polyolefin surface has proven unsatisfactory; and so have attempts to provide an intermediate adhesive layer.
A large portion of the favorable economics of chlorinated vinyl chloride-based pipe, that is, CPVC/PVC, derives from its solvent-cementability. It is this property which is sought in flexible extruded pipe because the overall cost of installation of such pipe is greatly decreased. However, to date, the significance of the fact that not all installations require the extreme flexibility of PEX pipe, has not been exploited. There are numerous installations in which the pipe needs to be bent no more than 90°, or, returned over an arc having a diameter in the range from 30.5 cm (1 ft) to 1.83 m (6 ft), which arc may be a function of the diameter of the pipe. The combination of these two considerations dictates that, in a twin-layered pipe in which a thin inner annular core is to be CPVC/PVC so as to protect an outer thick tubular layer from oxidative degradation from within the pipe, the elastomer for that outer layer remains to be chosen.
It is known that certain segmented thermoplastic copolyester elastomers (“COPE”) commercially available under the trademark Hytrel® are flexible at low temperatures, tough and resilient, with good impact strength and flex fatigue, high resistance to creep and good retention of these properties at elevated temperatures and resistance to deterioration from many industrial chemicals, oils and solvents (see brochure titled “DuPont Hytrel® polyester elastomer—Extrusion Guide”). The brochure states “Experience has shown that Hytrel® is extremely compatible with most rigid and flexible PVC compounds, and equipment normally used to coextrude rigid and flexible PVC has given good results with Hytrel®. The lower melting point grades of Hytrel® generally give best results”. The brochure refers to both rigid and flexible PVC, apparently overlooking that flexible PVC is plasticized, and the plasticizer migrating to the boundary between the PVC and Hytrel®, could eventually cause delamination. Moreover, the brochure fails to identify those grades of Hytrel® which are extremely compatible. However, since measured melting points of Hytrel® resins range from about 150° C. to 300° C., it is reasonable to ascertain that lower melting point grades are those in the range from about 150° C., but lower than 200° C. Nor does the brochure suggest how hydrolytically stable any particular Hytrel® may be over many years subjected to hot potable water under pressure; nor what tensile stress would be required for pipe carrying that potable water; nor that any one or more particular Hytrel® elastomers might provide a pressure design basis of 1380 kPa (200 psi) @ 82° C. for pipe which meets SDR-11 (standard dimension ratio) pipe dimensions, when tested in accordance with ASTM D-2837.
Nor does the brochure suggest which Hytrel® might be weldable or fusible to CPVC/PVC without substantially sacrificing flexibility of the laminate, and whether the laminate would not delaminate under operating conditions. Since the term “flexible pipe” is relative, when the term is used herein to define pipe which is CPVC/PVC, the term defines and refers to the ability of a pipe having a nominal diameter of 1.9 cm (0.75″) to bend through an angle of at least 90° while meeting SDR-11 requirements, without damaging the integrity of the wall of the pipe.
Most of all, the brochure fails to suggest that even if one or more Hytrel® elastomers were found which met the requirement of minimum pressure design basis and tensile stress under the stated conditions, even those Hytrel® elastomers would be found to be susceptible to hydrolytic degradation. Such degradation is found to be exacerbated by oxidizing agents in potable water, after service for several thousand hours but less than the expected service life of 50 years.
A portion of the many “COPE” sold under the Hytrel® trademark are segmented copolyester elastomers disclosed in U.S. Pat. Nos. 3,023,192; 3,651,014; 3,763,109; 3,766,146; 3,784,520; 3,801,547; 4,264,761; inter alia. Useful COPE elastomers are also commercially available from producers other than E. I. duPont and disclosed in U.S. Pat. No. 4,156,774 to Ciba-Geigy; U.S. Pat. No. 4,349,469 to Eastman Kodak; U.S. Pat. Nos. 4,355,155 and 4,405,749 to GAF.